Assumed to be English by default. ConfigEntityStorage will set an
appropriate language when creating new entities. This default applies to
imported default configuration where the language code is missing. Those
should be assumed to be English. All configuration entities support third
party settings, so even configuration entities that do not directly
store settings involving text in a human language may have such third
party settings attached. This means configuration entities should be in one
of the configured languages or the built-in English.